Films composed principally of a PAS such as poly(phenylene sulfide) which may hereinafter be called "PPS" have excellent properties such as high heat and chemical resistance and are useful for various industrial applications. Their unstretched films cannot however avoid deformations such as heat shrinkage in a high temperature range. Unstretched PAS sheets of smaller orientation are hence used for applications where high-temperature dimensional stability is required. PAS has a lower glass transition temperature and its crystallization is indispensable in order to impart heat resistance to them. Among conventional unstretched PAS sheets, uncrystallized ones cannot however show sufficient strength when the temperature increases to 200.degree. C. or so, while crystallized ones have a low elongation and are very brittle at that temperature.
Crystallization of an unstretched PAS film is effected by a heat treatment. Conventionally, a sheet-like (or film-like) formed product composed principally of a PAS such as PPS has heretofore been produced by melting the starting resin, extruding the melt through a slit die, cooling and solidifying the extrudate into a sheet, and then subjecting the sheet to a heat treatment. The heat treatment may be conducted after application of stretching if necessary. This conventional heat treatment is conducted by bringing a sheet, which is to be treated, into contact with a heated liquid or gas stream or a surface of a heated solid such as a roll (Japanese Patent Publication No. 42611/1984). It has also been known to smoothen the surfaces of a sheet-like material by subjecting the sheet-like material to a heat treatment while supporting it with clamps or the like at its periphery or causing it to continuously pass through a hot-air oven in a state supported at one or two points continuously or heat treating it on a smooth stainless steel belt, followed by compression forming or pressing between pressure rolls (Japanese Patent Laid-Open No. 184619/1984).
These conventional heat treatment methods are however difficult to provide sheets excellent in both planarity and smoothness when heat-treated sheets are industrially produced from PAS. Moreover, the provision of a smooth sheet requires a complex step such as compression forming or rolling, so that larger production facilities are required.
Incidentally, the behavior of a PAS sheet upon its heat treatment includes that a sheet cooled and solidified in an amorphous state is exposed to a temperature above the glass transition point owing to its heating and upon a lapse of a predetermined time, is crystallized and hardened. When a PAS sheet is subjected to a heat treatment in a heated liquid or gas stream by way of example, the sheet expands and becomes sticky as the temperature increases. When the temperature increases beyond the glass transition point of the PAS and the sheet becomes soft, the sheet is distorted or locally elongated, sticks to another material or object which is in contact with the sheet, or forms a roughened surface due to eruption of low boiling materials contained inside the PAS. Crystallization thereafter proceeds, thereby hardening the sheet. The resultant sheet is however poor in smoothness and its surface conditions are inferior. In the heat treatment method in which a PAS is simply brought into contact with a surface of a solid such as a heating roll or stainless steel belt, the sheet expands and moreover becomes sticky as the temperature increases. Accordingly, the sheet may locally and slightly float from the surface of the solid. Subsequent crystallization results in hardening of the sheet. In this case, height differences arise in the surface of the sheet between areas maintained in contact with the solid and those floated from the solid. It is hence only possible to obtain a sheet inferior in planarity and smoothness. When a PAS sheet is subjected to a heat treatment while holding it with clips or the like, the clipped parts become useless and moreover, the resultant sheet is susceptible to breakage from the clipped parts.
Even if such a crystallized sheet of poor planarity and smoothness is pressed by compression forming or rolling, it is impossible to fully remove the thickness irregularity, warpage, small ruggedness and the like to make the sheet excellent in planarity and smoothness because it has already been crystallized. Moreover, the process is complex and disadvantageous economically.